Status of Dorper sheep as hosts of ectoparasites
L.J. Fourie
a,*, I.G. Horak
baDepartment of Zoology and Entomology, University of the Orange Free State, P. O. Box 339, Bloemfontein 9300, South Africa
bDepartment of Veterinary Tropical Diseases, Faculty of Veterinary Science, University of Pretoria, Onderstepoort 0110, South Africa
Abstract
Seventeen ixodid tick species or subspecies have been collected from Dorper sheep in South Africa. The diseases transmitted or the conditions caused by these ticks in sheep are discussed. Although Dorper sheep may be infested with the scab mite,Psoroptes ovis, this does not seem to cause production losses. These sheep may, however, serve as a source of mite infestation for woolled sheep. The majority of Dorper sheep are infested with the larvae of the nasal bot ¯y,Oestrus ovis, they may also be subject to myiasis caused by the larvae of two calliphorid ¯ies. In certain regions of the country Dorper sheep are prone to serious ¯y worry by femaleSimulium chutteri. Although Dorper sheep may be infested by various louse species these do not seem to reach large numbers or cause production losses.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Dorper sheep; Ixodid ticks; Psoroptes ovis; Lice; Myiasis; Simulium chutteri; Production losses
1. Introduction
Sheep form a critical component of most livestock production systems in every continent. According to recent FAO estimates the world sheep population numbers 1,070 million and with the increase in human population numbers of small ruminants are also increasing. The various sheep breeds are able to survive and be productive in a wide range of ecotypes, in many cases exploiting the scarce feed resources available. The Dorper sheep, which was recognised as a breed in 1950, has shown an unparalleled increase in numbers in South Africa. According to Marais and
Schoeman (1990) this is because this breed met a very real need for a fertile and productive mutton breed for the extensive and arid grazing areas in much of the country. However, there is a continual need to improve management and reduce the impact of major limiting factors affecting productivity for every breed of live-stock. One such limiting factor is ectoparasites and the diseases they transmit. Although sheep may be infested with a wide diversity of ectoparasites many of these may have no, or little in¯uence on produc-tivity. This paper reviews some of the ectoparasites which occur on Dorper sheep in South Africa. Some of the arthropods (¯ies) referred to may not fall into the traditional de®nition of an ectoparasite but are never-theless included because of their potential impact on productivity. Most of the available information deals with ticks and hence the main focus of this paper is on these ectoparasites.
*Corresponding author. Tel.:2751-4012371;
fax:2751-4461662.
E-mail address: [email protected] (L.J. Fourie)
2. Ticks infesting dorper sheep
World-wide sheep are known to be infested by about 97 tick species belonging to 10 genera (Liebisch, 1997). These ticks and the diseases they transmit are of considerable economic and social importance to commercial enterprises and individual livestock owners. Besides the pathogens and toxins they transmit, ticks can also cause direct damage to the host including blood loss anaemia and skin wounds as well as lameness due to foot abscesses or large infes-tations of ticks feeding on the feet. The impact of tick infestations on production losses in sheep is, however, poorly understood and has received little attention from the scienti®c community (Uilenberg, 1997).
The results of tick surveys covering the major production areas of Dorper sheep in South Africa, have shown that 17 different tick species or subspecies belonging to six genera infest this breed (Horak et al., 1991a; Horak and Fourie, 1992) (Table 1). Of these all developmental stages (larvae, nymphs and adults)
of six tick species (A. hebraeum, B. decoloratus,
H. silacea, R. appendiculatus, R. evertsi evertsi and
R. glabroscutatum) feed on Dorper sheep. In the case of A. marmoreum only immature ticks occur on the sheep and for 10 other species only the adults feed on sheep (Table 1). Seven of the species which infest Dorper sheep can either transmit pathogens or cause direct damage to sheep (Table 2).
2.1. Amblyomma hebraeum
This tick is distributed mainly in the higher rainfall regions in the northern and northeastern provinces of South Africa and also along the eastern coast as far as Humansdorp (Walker, 1991). The larvae and nymphs utilise a large variety of small and large mammals and some ground-nesting birds. The adults prefers large hosts such as cattle, kudu, eland, buffalo and giraffe (Horak et al., 1987a,b).
In South AfricaA. hebraeumis the main vector of
Cowdria ruminantium, the cause of cowdriosis (heart-water) in sheep, goats and cattle (Bezuidenhout, 1987). Sheep may not only act as a source of ticks but also ofC. ruminantiumfor goats or cattle and vice versa (Horak et al., 1991a). Data on exact impact of
Table 1
Ticks infesting Dorper sheep in South Africaa
Tick species Larvae Nymphs Male Female Province Reference
Amblyomma hebraeum E.C. Horak et al. (1991a)
A. marmoreum E.C. Horak et al. (1991a)
Boophilus decoloratus E.C.; W.C.; F.S. Horak et al. (1991a); Horak and
Fourie (1992)
Haemaphysalis silacea E.C. Horak et al. (1991a)
Hyalomma m. rufipes E.C.; F.S. Horak et al. (1991a); Horak and
Fourie, 1992; Fourie and Kok, 1995
H. m. turanicum E.C.; N.C. Horak and Fourie (1992)
H. truncatum N.C.; W.C.; E.C.; F.S. Horak and Fourie (1992)
Ixodes rubicundus N.C.; W.C.; E.C.; F.S. Horak and Fourie (1992)
Rhipicephalus appendiculatus E.C. Horak et al. (1991a)
R. evertsi evertsi N.C.; W.C.; E.C.; F.S. Horak et al. (1991a); Horak and Fourie (1992)
R. exophthalmosb E.C. Horak et al. (1991a); Horak and
Fourie (1992)
R. follis E.C.; F.S. Horak and Fourie (1992)
R. gertrudae N.C.; W.C.; E.C.; F.S. Horak and Fourie (1992)
R. glabroscutatum E.C.; W.C. Horak et al. (1991a); Horak and
Fourie (1992)
R. neumanni N.C.; E.C. Horak and Fourie (1992)
R. simus E.C. Horak et al. (1991a)
R. nearR. punctatus F.S. Horak and Fourie (1992)
cowdriosis on Dorper sheep production are lacking. When viewed against the geographical distribution of
A. hebraeum it is likely to be limited. It has been reported (Alexander, 1931) that Persian crossbred sheep have a strong natural resistance toC. ruminan-tium.
2.2. Hyalomma spp
The two species of this genus found in South Africa prefer the drier regions of the country (Howell et al., 1978). The immature stages prefer scrub hares as hosts (Horak and Fourie, 1991) whereas the adults prefer large herbivores, such as cattle, eland and zebras (Horak et al., 1991a, b). Where sheep are farmed in regions in which these ticks abound they are also good
hosts for the adults (Horak and Fourie, 1992). H.
marginatum ru®pesand H. truncatumattach mainly to the anogenital/inguinal region of Dorper sheep (>63%) (Fourie and Kok, 1995) and clusters of ticks in excess of 50 are not uncommon (Kok and Fourie,
1995). In the case of H. truncatum a signi®cant
proportion of the ticks may also attach to the feet
(Fourie and Kok, 1995).Hyalomma spp, because of
their long mouthparts, cause wounds that may be invaded either by the larvae of blow¯ies (Chrysomya
spp) or by bacteria, resulting in the formation of abscesses, lameness and footabscess.
2.3. Ixodes rubicundus
This tick, known as the Karoo Paralysis tick, is widespread in the southern, central and eastern Free State, the Karoo regions of South Africa and in a number of districts in the Western Cape Province. In
other provinces its distribution is more con®ned (Walker, 1991). The preferred hosts of the immature stages are rock elephant shrews, red rock rabbits and caracals (Fourie et al., 1992; Horak and Fourie, 1992). Sheep, goats and cattle (Spickett and Heyne, 1988; Fourie et al., 1989) as well as wild ungulates (Fourie and Horak, 1987) can be paralysed by a toxin pro-duced byI. rubicundusfemales. Of all the tick species infesting Dorper sheep the Karoo paralysis tick is most likely to cause the greatest production losses. If no prophylactic measures are taken during years of high abundance, in excess of 50% of a ¯ock may become paralysed and suffer mortality if not treated (Fourie; unpublished data). The chances of a sheep becoming paralysed is related to infestation density (Fourie et al., 1989). In a comparative study it was shown that Dorper sheep are more at risk of becoming paralysed than Merino sheep (Fourie and Kok, 1996). This is not necessarily related to a greater susceptibility to the toxin but rather to generally higher tick burdens on the Dorper sheep. The differences in tick burdens between the two sheep breeds are possibly related to differing grazing patterns and diet selection. The feeding beha-viour of the host plays a major role in determining tick/host sympatry and hence the chances of a host
becoming infested with I. rubicundus (Fourie and
Kok, 1992).I. rubicundusis con®ned mainly to hilly or mountainous terrain (Fourie et al., 1991).
2.4. Rhipicephalus evertsi evertsi
Of all the tick species infesting Dorper sheep R.
evertsi evertsimost probably has the widest distribu-tion. The preferred hosts of all stages of development are equids and eland (Horak et al., 1991b) but sheep
Table 2
Diseases of sheep associated with tick infestation
Tick species Disease Reference
Amblyomma hebraeum Heartwater, foot abscess Bezuidenhout (1987); MacIvor
and Horak (1987)
Hyalommaspp Foot abscess and lameness Fourie and Kok (1995)
Ixodes rubicundus Karoo paralysis Fourie and Kok (1996)
Rhipicephalu Evertsi evertsi Theileria separata(asT. ovis), spring lamb paralysis
Jansen and Neitz (1956); Hamel and GoÈthe (1987)
Rhipicephalus glabroscutatum Foot abscess MacIvor and Horak (1987)
Rhipicephalus neumanni Lameness Walker (1990)
are also ef®cient hosts of the adults (Horak et al., 1991a). Engorging female ticks can cause paralysis in adult sheep (Hamel and GoÈthe, 1987), but this usually occurs in young, spring-born lambs, hence the name spring-lamb paralysis (Howell et al., 1978).R. evertsi evertsi is also a vector ofBorrellia theileri causing spirochaetosis in sheep (Howell et al., 1978) and of
Theileria separata(asT. ovis)causing theileriosis in sheep (Jansen and Neitz, 1956).
2.5. Rhipicephalus glabroscutatum
This species occurs in the Eastern Cape Province as well as in areas of the Western Cape Province (Walker, 1991). Kudu are the preferred hosts of all stages of
development of R. glabroscutatum while cattle and
Angora goats are better hosts than are Dorper sheep (Horak and Knight, 1986). Large numbers of larvae appear to die or detach before moulting to nymphs on these sheep (Horak et al., 1991a). AdultR. glabros-cutatumattach around and between the claws of their host's hooves and are a major contributory cause to foot abscesses, and hence, lameness in small livestock (MacIvor and Horak, 1987).
2.6. Rhipicephalus neumanni
This tick occurs in scattered localities in the Karoo (Walker, 1991) and sheep are one of its most com-monly recorded hosts (Walker, 1990; Horak and Fourie, 1992). Adults attach between the claws of the hooves and are a contributory cause of lameness (Walker, 1990).
2.7. Rhipicephalus sp. near punctatus
This species has only been recorded in the central, southern and southwestern Free State and adjacent regions of the Northern Cape Province. The preferred hosts of all stages of development are scrub hares, while the immature stages also feed on rock elephant shrews and the adults on goats (Fourie and Horak, 1991; Horak and Fourie, 1991; Fourie et al., 1992).
This tick has been associated with paralysis in Angora goat kids in the southwestern Free State (Fourie et al., 1988). It is not known whether this tick can cause paralysis in lambs of Dorper sheep.
3. Sheep scab
Sheep scab is caused by the non-burrowing, astig-matid mitePsoroptes ovis. It is one of the oldest sheep diseases known to man, and was regarded as a serious
disease as early as 180 B.C. Feeding mites causes
intense irritation and the sheep become pre-occupied with scratching, cease to feed and rapidly become emaciated (Kirkwood, 1986). Mite feeding is not the direct cause of lesion formation, but sheep scab is a form of allergic dermatitis initiated by the mites faeces. The heat and humidity produced by the in¯am-matory reaction results in the micro-climate needed for mite survival and the leakage of serous exudate the basis of the mites nutrition (Bates, 1996).
The disease, which occurs commonly in South Africa, is believed to be out of control. Although sheep scab was previously thought to be a winter disease only, recent research (Fourie, unpublished data) has shown that it can occur in South Africa during winter and summer on both Merino and Dorper sheep. There is, however, a profound difference in the scab symptomatology between Dorper and Merino sheep. Comparative studies have shown that lesion size is signi®cantly greater in Merino sheep, while the growth of the lesion is signi®cantly slower in Dorper sheep (Fourie and Meintjes, unpublished data). Whereas infested Merino sheep lost 30% of their initial body mass over a 16 week period Dorper sheep maintained or increased their body mass. These dif-ferences are believed to be related to the fact that the density of hair follicles is less in Dorper compared to Merino sheep resulting in a less favourable micro-climate for the propagation and survival of the mite (Fourie and Meintjes, unpublished data).
As is the case with ticks the ability of sheep to develop resistance toP. ovishas been poorly studied. From a control perspective the slow growth of lesions and the sub-clinical nature of sheep scab on many Dorper sheep, make detection dif®cult. Because of this it is thought that Dorper sheep could be important disseminators of the disease in non-infested ¯ocks or regions.
4. Flies and bot ¯ies
Metallic blow¯ies belonging to the generaLucilia
caus-ing myiasis in sheep in South Africa (Howell et al.,
1978). Lucilia cuprina is a primary blow¯y and is
mainly responsible for myiasis in woolled sheep breeds (Leipoldt, 1996). Blow¯y strike in Dorper sheep caused byL. cuprinais very rare and has only been observed in those sheep with a high wool com-ponent, most probably the result of previous crossings with Merino sheep. Myiasis caused by the secondary
blow¯yChrysomya albicepshas been observed more
frequently in Dorper sheep and is related to abscesses caused by tick feeding or wounds resulting from other factors (Fourie, unpublished data).
Female black¯ies (Simulium chutteri) are known
to annoy livestock along the Orange River system. It has been conjectured that at certain times of the year the annoyance levels can be such that the condition of especially small livestock can be detrimentally affected. Decreased lambing percentage and occasional deaths have also been reported (Jor-daan and Van Ark, 1990). In a comparative study between Dorper and Merino sheep there were no signi®cant differences in the attraction of black¯ies to either breed. However, signi®cantly fewer females engorged on Dorper than on Merino sheep (Kok et al., 1994).
The sheep nasal bot ¯y (Oestrus ovis) has a world-wide distribution wherever sheep and goats are kept and occurs in most parts of South Africa (Howell et al., 1978). It has previously been shown that infestations with the larvae ofO. ovisretards the live mass gains of Merino lambs (Horak and Snijders, 1974). In a com-parative study between Merino and Dorper sheep it was found that the percentage of sheep infested and the level of infestation is practically the same for the two breeds (Horak, 1977). It can, therefore, be pre-sumed that infestation of Dorper lambs with the larvae ofO. oviswill affect live mass gain.
5. Lice
Sheep in South Africa may be infested with three sucking louse species (Linognathus africanus,L. ped-alis and L. ovillus) and two biting louse species (Damalinia ovis and D. peregrina) (Howell et al.,
1978). Biting lice (D. ovis) can cause production
losses in wool sheep breeds (Howell et al., 1978). It is, however, perceived that lice infestation may have
little or no effect on the production of Dorper sheep, and reports of heavy louse infestations are lacking. Besides the above mentioned lice Dorper sheep may also carry small burdens (nymphs and adults) of the Angora goat louseD. limbata, speci®cally where these two breeds are farmed together and are in close contact (Horak, unpublished data). Previous observa-tions, have, however, shown that such infestations are not permanent.
6. Host resistance
Numerous reports on the resistance of sheep to various ectoparasitic infestations are based on anec-dotal evidence and adequately quanti®ed studies are generally lacking. Although it has been reported that certain sheep breeds can develop resistance to the feeding of particular tick species (Tatchell, 1997) few well documented reports are available. Experi-mental infestations of crossbred Dorper and Merino
sheep with A. hebraeum adults over a 23 period
produced a small reduction (20%) in tick numbers form acquired resistance (Norval et al., 1988). With
speci®c reference to I. rubicundus ®eld observation
have also shown that Dorper sheep may become paralysed more than once within a speci®c season (Fourie, unpublished data) indicating that acquired resistance to tick feeding may not be well established. Clearly more research is required to quantify host resistance in Dorper sheep against diseases and the feeding of ticks. Through selection, it should, how-ever, be possible to breed Dorper sheep displaying a high degree of resistance to certain diseases and ectoparasitic infestations.
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